In recent years there has been a continuing effort to produce crude oil and gas from subterranean formations located in ever-increasing water depths. One approach to producing hydrocarbons from deepwater fields is to locate the wellheads on the seafloor using subsea production equipment. However, as the water depth exceeds the capability of divers, accessing the wellheads for servicing and workovers becomes more difficult and more costly, resulting in the need for submarines, remotely operated vehicles, or the like.
Another approach to producing hydrocarbons from deepwater fields is to extend the well casing strings through the water column and locate the wellheads above the water surface, commonly called surface wellheads. Surface wellheads are generally preferred for deepwater offshore production because they have less complex drilling and production equipment and reduced maintenance costs. However, the structure required to support surface wellheads becomes increasingly expensive as water depth increases.
During the past few years there have been a number of developments in deepwater oil and gas production technology, including the semisubmersible floating production platform. The semisubmersible floating production platform consists of a flotation hull and deck. The flotation hull typically has four or more large diameter vertical columns which extend downwardly from the deck and are supported on two or more horizontal pontoons. The flotation hull, when de-ballasted, allows the platform to be floated to the drill site. At the site, the hull is ballasted with seawater such that it becomes partially submerged, with the platform deck remaining above the water surface. The semisubmersible platform is held in position using mooring lines. Submerging the flotation hull beneath the water surface reduces the effect of environmental forces such as wind and waves and results in a relatively stable work deck. However, while stable for most drilling and production operations, the semisubmersible platform still responds to the environment to an extent such that surface wellheads are unattractive due to the complexity and cost of the riser tensioner and other clearance systems required to permit relative motion between the platform and foundation. Instead, relatively complex and costly subsea production equipment is typically used with the semisubmersible platform to produce hydrocarbons from semisubmersible floating production platforms.
Tension leg platforms ("TLP's") are another technology used to produce hydrocarbons in deep water. The TLP is a platform for drilling and production operations that is moored to the seafloor using stiff, vertical tethers (also commonly called "tendons"). The TLP hull and deck, which together comprise the platform, are similar in configuration, construction, and hydrodynamic properties to the semisubmersible floating production platform. The hull provides excess buoyancy to support the deck and to tension the tethers and production risers. The deck supports drilling and production facilities. Mooring the platform using stiff, vertical tethers, which are tensioned by the excess buoyancy of the hull, virtually eliminates heave, roll, and pitch motions. As a result, a heave-restrained platform is provided which permits surface wellheads to be used with all of their operational benefits. Heave restraining the entire platform, including the drilling rig, crew's quarters, and ancillary production equipment, requires a substantial amount of additional buoyancy and tether steel, thereby increasing the overall cost of the TLP to a point which is great compared to the operational benefit gained.
Because most of the petroleum drilling and production equipment typically used on an offshore platform is not greatly affected by heave, it would be desirable to eliminate many of the difficulties associated with producing deepwater oil fields by combining the heave restraining benefit of the TLP with the reduced cost benefit of the semisubmersible floating production platform by heave restraining only the surface wellhead equipment.